1. Field of the Invention
The present invention relates to a fixing apparatus used with an image forming apparatus such as an electrophotographic copying machine, an electrophotographic laser beam printer and the like, and a storage medium for storing a program for carrying out a cleaning process of the fixing apparatus which can be read by a computer.
2. Related Background Art
In the past, as fixing apparatus used with an image forming apparatus such as an electrophotographic copying machine, an electrophotographic laser beam printer and the like, a fixing apparatus of heat roller type and a fixing apparatus of on demand type have been proposed. In the fixing apparatus of heat roller type, a pair of roller (fixing roller and a pressure roller) having a heating device are urged against each other to form a nip therebetween and, by passing a sheet as a recording material (such as a paper sheet) through the nip, the toner on the sheet is fused, thereby fixing the toner to the sheet.
Among the pair of rollers, the fixing roller contacted with the front surface of the sheet (bearing the toner) is a cylindrical roller having a surface made of material having good mold releasing ability and has a halogen heater (heating device) therein for heating the toner. On the other hand, the pressure roller contacted with a rear surface of the sheet is constituted by a core cylinder and an elastic layer coated on the core cylinder and serves to pressurize the toner layer appropriately.
Although it is ideal that all the toner on the front surface of the sheet is thermally fused to be fixed to the front surface of the sheet, if there are cold offset toner which was not completely fused, hot offset toner which was fused excessively or/and toner which was offset to the fixing roller electrostatically (referred to as "toner contamination" herein after), such toner contamination is adhered to the surface of the roller (fixing roller or pressure roller) which has lower mold releasing ability.
When the fixing roller has lower mold releasing ability than that of the pressure roller, the toner contamination is adhered to the fixing roller. In this case, since the fixing roller is always heated to the toner fusing temperature during image formation, the toner contamination is in a fused condition. Thus, since the toner contamination is shifted to a next sheet by mixing with a toner image on the next sheet, it is hard that the fixing roller is continuously contaminated. However, rarely, the toner contamination remains on the surface of the fixing roller. In such a case, the image on the sheet may be contaminated.
On the other hand, when the pressure roller has lower mold releasing ability than that of the fixing roller, the toner contamination which was offset to the fixing roller once is transferred to the pressure roller. Since the temperature of the pressure roller is lower than that of the fixing roller, the transferred toner contamination is not always completely fused on the pressure roller. Further, the pressure roller is not contacted with the toner image on the front surface of the sheet, the toner contamination is not entrained by the toner image, with the result that the toner contamination is accumulated on the pressure roller. If the toner contamination is accumulated on the pressure roller greatly, since the mold releasing ability of the pressure roller is decreased, the sheet will be adhered to the pressure roller or the accumulated toner contamination will be transferred to the rear surface of the sheet at once, thereby contaminating the sheet.
In the fixing apparatus of on demand type, in place of the halogen heater and the fixing roller of the fixing apparatus of heat roller type, a heater such as a ceramic heater and a thin film made of polyimide are used to reduce heat capacity of the fixing apparatus, thereby permitting quick start and saving energy or power.
In such a fixing apparatus of on demand type, since the heat capacity is small and temperature response is excellent, it is not required to preheat the fixing apparatus and fine temperature control can be effected, and the energization of the fixing apparatus can be turned OFF when the sheet is not passed.
However, in the fixing apparatus of on demand type, if the above-mentioned temperature control is effected, since the pressure roller is not heated when the sheet is not passed, the temperature is hard to be increased in comparison with the fixing apparatus of heat roller type (about 100.degree. C. at the maximum). Thus, the toner contamination offset to the fixing roller and transferred to the pressure roller is not fused on the pressure roller and remains on the pressure roller in a cured condition.
Since such a condition is existed, even when the cleaning is effected by using a cleaning paper disclosed in Japanese Patent Application Laid-Open No. 3-58074 (1991) (i.e., a sheet on which a solid image was fixed), the cured toner on the pressure roller cannot be cleaned. On the contrary, in some cases, the solid image on the cleaning paper may be stripped by the cured toner on the pressure roller to promote the toner contamination on the pressure roller.
Incidentally, in both of the fixing apparatus of heat roller type and the fixing apparatus of on demand type, when the cleaning is effected by using the cleaning paper, it is required that the cleaning paper on which the solid image was previously formed is supplied in a manner reverse to the normal sheet pass (i.e., a manner in which the solid image is to be contacted with the pressure roller). Thus, the cleaning operation is troublesome or erroneous cleaning operation may occur.